I am an only child, and am childless by choice. I don't think I can narrow in on the stupidest thing I've heard, but I can give you a list to choose from.
To my parents, because I was an only child:
"It's a shame you didn't have more." Hurtful, because my mother couldn't have another child.
"She must be lonely." Do people think I spent my childhood locked in a box by myself?
"Why didn't you adopt?" Very personal. None of your business.
"It's a shame she'll never have a brother or sister." My father was an only child, so this was doubly stupid.
"It must be nice to have some peace and quiet at home." Yes, it is nice to have a child who isn't a screaming heathen, because we took the time to teach her manners, and the value of silence, instead of letting her run wild. You should try that.
To me, because I don't have kids:
"You only really know what love is when you have a child." Possibly the most selfish reason I've ever heard put forth for having kids.
"Did you never want children?" None of your business, and if I did want them, wouldn't this be a painful question to answer?
"You would have made such a good mom." No, I wouldn't, which is why I chose not to have kids. More people should acknowledge this, and make this choice.
"Why would you choose not to have kids?" None of your business. This is never anyone's business but the person or couple involved.
"You're depriving us of the experience of being grandparents." Obviously, from my own mother. I told her she'd better get to work on having another kid if she expected to be a grandparent. I'm still not clear on the logic of having a child just to provide someone else with the experience of being a grandparent.
Shall I go on?